In Table REF  we list the major classes of materials currently included in the database. The materials are grouped according to their prototype, see Section REF . For each prototype we list the corresponding space group, the total number of materials, and the number of materials satisfying a range of different conditions. The atomic structure of some of the different prototypes were shown in Figure REF . The vast majority of the 2D materials that have been experimentally synthesised in monolayer form are contained in the C2DB (the 55 materials in Figure REF  in addition to seven metal-organic perovskites). These materials are marked in the database and a literature reference is provided. Additionally, 80 of the monolayers in the C2DB could potentially be exfoliated from experimentally known layered bulk structures[1]}, [2]}, [3]}, [4]}. These materials are also marked and the ID of the bulk compound in the relevant experimental crystal structure database is provided.
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